“At the time John just thought it was somebody interested in unsolved crimes.”

John, who wants to remain anonymous, recalls other names mentioned by Black. They included Mary Boyle, who disappeared, aged six, in Cashelard, ­Ireland, 37 years ago. Black is known to have regularly travelled to ­Ireland.

Black was convicted in May 1994 of ­murdering ­Susan Maxwell, Caroline Hogg, five, of Edinburgh, and Sarah Harper, ten, in Leeds.

In 2011 he was convicted of ­abducting and murdering Jennifer Cardy, nine, in Northern Ireland.

Black, who is in Wakefield jail, West Yorks, also spoke to John about ­missing ­children whose plight never made ­headline news, including Suzanne ­Lawrence, 14, of Harold Hill, Essex, who has been missing since 1979.

Another was Christine Markham, nine, who disappeared on her way to school in Scunthorpe in 1973.

Mr Clark said: “Clearly, Black knew a lot about the cases when others did not.”

He has put together a dossier of evidence and passed the information about Genette Tate to Devon and ­Cornwall police.

A detective on the cold case team has spoken to him. The former intelligence officer said: “I believe Black was SDHp responsible for some or all of these ­murders.”